1. Field of the Invention
The present invention relates to an impact tool for simultaneously providing a rotational force and an impact force to an object.
2. Disclosure of the Prior Art
In the past, an impact tool for providing a rotational force of an output shaft to an object, and simultaneously giving an impact force to the object through the output shaft has been used to drill concrete, brick, stone and so on, which is also called as hammer drill.
For example, Japanese Patent Gazette No. 2595262 discloses a hammer drill comprising a motor, output shaft rotated by the motor and having a tool holder for detachably holding a tool, hammer for intermittently providing an impact force to the output shaft, and a piston for movably holding the hammer therein, and an impact force generator for converting an output of the motor into a reciprocating motion of the piston. An air chamber defined between the hammer and an inner bottom of the piston functions as an air spring to accelerate the hammer toward the output shaft. In addition, since this hammer drill has a gear shifter for automatically switching a reduction ratio between a slow-speed, high torque mode and a high-speed, low torque mode according to a load applied to the tool, the drilling operation can be efficiently achieved.
In addition, Japanese Patent Early Publication [kokai] No. 2004-082557 discloses a hammer drill comprising a motor, output shaft having a tool holder for detachably holding a tool and rotated by the motor through an intermediate shaft, hammer for intermittently providing an impact force to the output shaft, piston for movably holding the hammer therein, impact force generator for converting the rotation of the intermediate shaft into a reciprocating motion of the piston, and an impact force controller for changing a gear ratio between the motor and the intermediate shaft to control a magnitude of the impact force. According to this hammer drill, it is possible to provide the large impact force when using a drill bit with a large diameter as the tool, and provide the small impact force when using the drill bit with a small diameter. Thus, the drilling operation can be stably performed by use of an appropriate impact force according to the kind of tools used.
By the way, when the object is made of a hard material, or a large bore is formed in the object, the impact tool having the capability of generating a larger impact force is needed. To further increase the impulse force, it is proposed to use a heavy hammer, increase the torque by use of a high power motor, and/or extend the moving distance of the hammer in the impact tool. However, there is a problem that these proposals lead to an increase in weight and/or size of the impact tool.